compute_hex.js

function parseHexColor(c) {
  var j = {};

  var s = c.replace(/^#([0-9A-Fa-f]{2})([0-9A-Fa-f]{2})([0-9A-Fa-f]{2})$/, function(_, r, g, b) {
    j.red = parseInt(r, 16);
    j.green = parseInt(g, 16);
    j.blue = parseInt(b, 16);

    return "";
  });

  if(s.length == 0) {
    return j;
  }
};

function colorDifference(a, b) {
  var a = parseHexColor(a);
  var b = parseHexColor(b);

  if(typeof(a) != 'undefined' && typeof(b) != 'undefined') {
    return "#" + (a.red - b.red).toString(16) + (a.green - b.green).toString(16) + (a.blue - b.blue).toString(16);
  }
};

alert(colorDifference('#FFFFFF', '#AABBCC')); // returns : "#554433"